1. Field of the Invention
This invention relates to a degaussing circuit of the type commonly used in cathode ray tubes (CRT""s), color televisions, color monitors and other like type devices. More particularly, this invention relates to an improved degaussing circuit having a degaussing coil and a single thermistor element with a positive temperature coefficient (PTC) or mono-PTC unit for dual voltage applications.
2. Background of the Invention
Rasters in all cathode-ray tubes (CRTs) are created by the horizontal and vertical sweeping movement of three electron beams. The electron beams emitted by red, green, blue (R,G,B) cathodes are controlled by deflection circuits whose magnetic fields are orthogonal to the direction of the electron beams.
The system of magnetic control, which enables tracing of the raster, has drawbacks because the electron beams are susceptible to changes because of earth and stray magnetic fields, which give off interfering beams. Also, terrestrial flux can reach 0.05 mT (0.5 Gs) in the absence of nearby magnetic structures, with a maximum of 0.2 mT (2 Gs) in the vicinity of steel-frame buildings or underground iron, nickel or cobalt ore deposits. Even more dense magnetic fields can be produced by interfering sources, such as unshielded loudspeakers, motors, and transformers located in direct proximity of the CRT. These kinds of interferences result in tainted, or, in more extreme instances, a complete loss of purity of the primary colors in the CRT. In order to maintain proper pixel excitation by the designated electron beams, various demagnetizing circuits have been utilized in all but the smallest picture tubes. A network that employs these thermistor devices, having positive thermal coefficient (also known asa posistor, or PTC) are connected in series with degaussing coils have been around for a number of years, and are still one of the more commonly used circuits for degaussing applications. However, these circuits that utilize PTC thermistors demonstrate poorly defined characteristics because the PTC thermistors are susceptible to variations in line voltage, load current, and thermal drift. The degaussing circuits produce considerable in-rush current, which in turn create strong electromagnetic fields on their own that are often able to disturb adjacent sensitive electronic equipment.
The basic operational principles of the PTC thermistor circuit are well understood, i.e., upon power-up, a large-magnitude, alternating inrush current generates a magnetic field, whose amplitude, during duration of the first few AC cycles, far exceeds the level of the surrounding stray magnetic fields. Then, the current and its associated flux are gradually reduced to almost nill and the process is terminated. The purpose of this procedure is to cycle through Bxe2x80x94H magnetic hysteresis loop of the aperture grill/shadow mask and other ferrous alloy materials of the CRT so that the alternating orientation and diminishing magnitude of the magnetic field vectors reduce remnant flux to the negligible value.
The Trinitrons, having their phosphorous pixels fashioned in a form of vertically elongated strip are more susceptible to vertically oriented parasitic magnetic vectors than to horizontal ones. This is because vertical flux causes horizontal deviation of electron beams. For the same magnitude of horizontally and vertically oriented flux, the latter produces more noticeable color impurities. Thus, to minimize horizontal landing offset, predominantly vertical oriented compensating fields should be generated by the degaussing coils. This dictates horizontal placement of the coils directly above and below the CRT. In practice, since mutual orientation of the television set and the terrestrial magnetic field can vary widely depending on TV spatial positioning and its geographical location, the degaussing coils are mounted at the back of the CRT cone, producing magnetic field vectors angled in reference to the aperture grill. Such orientation of the degaussing coils also boost eddy currents induced in the magnetic shields that cover the back conical side of the CRT.
Further, calculations of magnetic flux produced by the degaussing coils are complicated because of the complex, three-dimensional geometrical form of the coils and their spatial orientation in reference to the CRT. Equations presented below illustrate simplified magnetic relations and, consequently, approximated results.
A vector relations of the scalar functions are represented by the following equations: A typical 67-turn degaussing coil that conducts the inrush current of 15 Apk produces magnetomotive force
MMF=I* N=15 Apk*67 trn=1,005 Apktrn
For a 32-inch CRT, each of the two quasi-rectangular degaussing coils is about 0.76 m wide and 0.28 m high.
Corresponding magnetic field intensity (H0) in geometrical center of the coil can be derived from the Biot-Savart""s law xcex4H=I*xcex4L*sin(r,dL)/4xcfx80r2 and converted to accommodate the rectangle-shaped inductor
H0=2IN[xxe2x88x922+yxe2x88x922]xc2xdpxe2x88x921=2*1,005 Apktrn*[(0.76 m)xe2x88x922+(0.28 m)xe2x88x922]xc2xd*pxe2x88x921=2,435 Apktrn/m
The magnitude of this field decreases in reversed proportion to the axial distance 1=0.25 m measured in direction normal to the plane of the coil, from its center to the aperture grill plane. Based on the same law, we have
H={IN/2p[(0.5x)2+(0.5y)2+12]xc2xd}*{x/[(0.5y)2+12]xc2xd+y/[(0.5x)2+12]xc2xd}
H={1,005 Atrn/2p[(0.38 m)2+(0.14 m)2+(0.25 m)2]xc2xd}*{0.76 m/[(0.14 m) 2+(0.25 m)2]xc2xd++0.28 m/[(0.38 m)2+(0.25 m2]xc2xd} greater than  greater than 1,100 Apktrn/m
Vector H constitutes a geometric composite of its horizontal and vertical fractions. The vertical element of that vector is reduced by the angled orientation of the coil that rests on the back side of the CRT. For the CRT having a deflection angle of a=110 deg, the inclination angle of the degaussing coil is approximately b=45 deg, thus
Hy=cos b*H=cos 45 deg*1,100 Apktrn/m=776 Apktrn/m
Finally, the vertical fraction of flux induced by the single coil can be found
By=m0mrHy=4pExp(xe2x88x927)H/m*1*776 Apktrn/m greater than  greater than 1 mT (10 Gs)
In spite of apparent simplicity of the demagnetization process, there are several challenging issues. The auto-volt or wide-range AC operation requires automatic adaptation of the degaussing circuit to the line voltage that may span from 88 to 288 volts (proportion in excess of 1:3). Since voltage regulation of the demagnetizing circuits is impractical, the only viable alternatives are switched PTC thermistors and/or current-limiting resistors. Networks based on switched PTC thermistors and/or resistor components reduce the current ratio to 1:1.7 for line voltage variations of (88-153)V or (176-288)V. Larger CRTs require stronger magnetic fields to maintain same flux density (number of magnetic lines per unit area) of the screen. For instance, a CRT enlarged from 27-inch to 36-inch CRT almost doubles its raster area. Magnetic flux has to be increased in the same proportion if the flux density is to remain unchanged. This can be accomplished only by increasing current flowing through the coil, the number of turns, or combination a of both. In either case, cost escalates steeply as increased current requires larger diameter wire, while increased number of turns command more length of wire.
For the former option (current), there are limitations of maximum current output from the residential AC outlet. Excessive surge whose I-t product exceeds that of the household circuit breaker poses risk of its activation. Large-magnitude current surges also induce undesirable voltage sags that may lead to malfunction of the associated entertainment system or sensitive computer hardware/software gear.
The latter alternative (number of turns) offers diminishing return on investment because coils having a larger number of turns exhibit increased impedance (both their resistive and reactive parts) that reduces current.
The third possible solution, combined simultaneous boost of current and turn numbers provide a proportional mix of advantages and limitations of these two methods.
By way of example, a CRT""s shadow mask can become magnetized by the earths magnetic field or by an electromagnetic field generated by the operation of neighboring devices or other electrical apparatus. Upon the shadow mask of the CRT being permanently magnetized, localized magnetic fields, which are produced, can effect the path of the CRT""s electro-beams and hence the overall picture quality is deteriorated. In addition, the color purity of the video image displayed by the CRT can be noticeably deteriorated. Color TV""s, computer monitors and the like include an automatic degaussing circuit in order to compensate for the presence of ambient magnetic fields. Such degaussing circuits are used with the CRT to be degaussed each time the power is supplied to the video display. A typical automatic degaussing circuit produces an ordinary demagnetizing field which decays in correspondence to an alternating decay current. One implementation of such a circuit may comprise a temperature sensitive device, for example, a positive temperature coefficient (PTC) thermistor connected in series between a dedicated degaussing relay and a degaussing coil. The degaussing relay is switched ON upon powering up of the CRT apparatus and the act of degaussing is completed within a few seconds after the power ON switch is turned on usually 1 to 2 seconds after the relay has been initialized.
It is an object of the present invention to provide a compact degaussing circuit that does not require a large positive temperature coefficient (PTC) element but can still demagnetize the desired circuit completely.
In one embodiment of the present invention there is a system for degaussing a cathode ray tube including an alternating current (AC) source; PTC element; a resistance; a first switch coupled between said AC source and said PTC element, which in turn is coupled to the resistance switching between a first position and a second position; a second switch coupled between the AC source and the PTC element switching between a first position and a second position; wherein in 110 volt operation, the first switch switched in a first position and the second switch switched in a second position; and conversely in 220 volt operation, the first switch switched in a second position and the second switch switched in a first position. Further, a first resistance is connected in series with a second resistance wherein both the first and second resistence being approximately of the same ohmic value wherein the resistance balancing current flow in the 110 volt operation and the 220 volt operation so as the current flow across the PTC element in both modes of operation being substantially equal.
In another embodiment of the present invention there is a degaussing circuit for a cathode ray tube including a source of alternating voltage potential; a first and second switch device coupled to the source; a single PTC element device coupled to the first and second switch devices; and a degaussing coil coupled to the PTC element device wherein a degaussing current flowing through the PTC element device equal to the current flow through the degaussing coil.
In still another embodiment of the present invention there is a degaussing system including a PTC element including a first leg and a second leg; a first coil connected to the first leg of the PTC element device; a second coil connected to the second leg of the PTC element device, wherein the current flow through the first coil and second coil being optimized such that the coil number being twice the usual number and the cross-sectional area being halved.
Yet still, in another embodiment the present invention, there is a degaussing circuit for an electronic device, including an alternating voltage potential; a driver device; a positive temperature coefficient (PTC) element device controlled by the driver device receiving current from said alternating voltage potential and connected to a degaussing device so that a degaussing current across the PTC element device is approximately the same as the degaussing current received by the degaussing device wherein said driver means operates in a first mode receiving 110 volts from the alternating voltage potential and in a second mode receiving 220 volts from the alternating voltage potential.
The features of the invention believed to be novel are set forth with particularity in the appended claims. The invention itself however, both as to organization and method of operation, together with further objects and advantages thereof, may be best understood by reference to the following description taken in conjunction with the accompanying drawing.